It's not about the amino acid profile of the muscle tissue as much as it is about how bioavailable the stuff is. And remember, 60% of the free amino acid pool in a human myocite is L-glutamine, the first amino acid to be lost during exercise. So, since chicken and its relatives don't have nearlly as much L-glutamine (or its close neighbor, glutamate) as beef (and bison, for that matter), it's not as "good" a protein for muscle recovery. Lycine and glycine are high on the list of important AAs as well.